It is known that monitoring water chemistry is essential to reliable, safe, and efficient operation of a reactor system. In particular, operators routinely collect data, on- and off-line, regarding the corrosive conditions caused in large part by the high operating temperature of and contaminants in the primary reactor water coolant.
In the context of a boiling water reactor (BWR), a Reactor Water Cleanup (RWCU) system maintains high reactor water quality by removing fission products, corrosion products, and other soluble and insoluble impurities. To remove undesirable chemical species and radioactive materials resulting from neutron activation of impurities and dissolved reactor materials, the RWCU may utilize a variety of means such as filtration and mitigation of corrosion. Generally, the water passes through isolation valves for redirection, heat exchangers for cooling, ion exchangers for undesired species removal, and the cleaned water is recirculated back into the reactor core.
Ideally, corrosion mitigation is performed precisely, selectively and locally, often by injecting hydrogen into reactor feed water. Hydrogen water chemistry (HWC) dilutes and counteracts oxidizing species, such as oxygen and hydrogen peroxide. Otherwise, these species may contribute to intergranular stress corrosion cracking (IGSCC) of susceptible materials, such as stainless steel reactor components.
Precise and selective mitigation is desirable because the level of mitigation that is appropriate for one reactor may be unwarranted for another, and the cost and negative impact of unnecessary measures are detrimental. For example, excessive HWC can generate unacceptable high radiation levels and doses.
Local mitigation is often desirable because flow rates and radiation levels vary at different locations within the reactor system, which impacts the amount of hydrogen necessary to achieve oxygen concentrations that will reduce the electrochemical corrosion potential (ECP) of materials within the reactor.
ECP sensors are commonly distributed to continuously monitor water chemistry at various sample sites throughout the reactor system, so that mitigation is tailored to actual conditions present in the system and to predict performance of specific reactor components. To measure oxygen concentration, for example, an ECP sensor is configured and positioned to function “in-line” with the flow at the sample site, such that reactor water flows into the sensor and around one or more ECP probes. ECP sensors are often placed in welding neck flanges in primary system piping (e.g., reactor recirculation lines, bottom head drain lines, and reactor cleanup lines). As used herein, the term “ECP sensor” refers to an ECP probe assembly that includes one or more ECP probes, and that may further include additional associated components, such as but not limited to, cables or wires, connectors, fittings, and encasements.
The typical BWR ECP sensor operates in a harsh environment—high-levels of radiation exposure, water flow rates of several meters per second, and water temperatures as high as 300° C. The primary threat to the ECP sensor is excessive turbulence at or flow rates around the sensor probes during RWCU operation. High water flow rates around the ECP probe may damage the probe and introduce debris from the probe into the reactor water recirculation loop. This debris is of particular concern because it has the potential to damage pump vanes, clog filter screen, or in the worst case scenario, become lodged in the fuel rod spacers and cause fretting of the fuel rod cladding. A secondary consequence of a damaged ECP sensor is that ECP measurements will be inaccurate or impossible, so sensor failure can yield a useful life that is far less than a single fuel cycle. Lastly, the geometry and orientation of the ECP probes may induce or increase vortex shedding and flow induced vibration in the RWCU pipes. Therefore, there is a need for systems and methods for preventing ECP sensor damage during reactor operation.